'94 W140 M119 limp mode when aux fans go to hi speed
 

This thing has be baffled. "94 S500 with 75K miles belonging to an elderly widow friend. I have been doing routine maintenance for the past two years. Last week while changing the AC blower regulator she tells me it has been stalling at times. Sure enough I eventually got it to act up. It appears to be going into limp mode. The throttle actuator is unresponsive until you reach the mechanical part of the throttle. I have ruled out the MAF and today while having a closer look I notice it occurred when the two aux cooling fans switched to high speed.
I'm thinking the added current draw created a low voltage so, I tried to duplicate it by turning the headlights on and rolling all the windows up to the stop at the same time. I could not make it fail doing this. Only when the aux fans go to hi speed.
Turning the engine off and restarting will reset it back to normal until the fans go to hi speed.
The M119 runs fine otherwise, never gets above 80c, seems smooth...

I'm stymied.

If this was posted on the Diesel part of the forum, there would be instant cries to sue the mechanic of 2 years because they apparently were the last person to touch it before it broke. ( and maybe even jimmied the blower reg to get more work. )

In reality, this is might be a case of bio degradable wiring insulation or a common ground with corrosion. Look for crumbling insulation on the engine / throttle body harness, the was common in the early 90's. Very odd things can occur when a ground point that has many wires leading to it becomes insulated from the body and one circuit back feeds another.

I fear it may be the harness insulation failing. Although, shutting it off seems to reset the thing. That seems to me to rule out any wires shorting but might be possible once the current draw of a device is removed. Say from a ground point as you mentioned.

And, the mechanic for the past two years has been pro bono. I'm afraid I don't have the time to replace a ETA or wiring harness.

The owner is 94 years old and I suspect it would be difficult adjusting to a different vehicle. I am hoping I can get a few more years out of this one.

Yea, I know that you get it. The sue happy people tend not to see when a shop / person genuinely helps someone out.

I'm also guessing that you are going to be torn between your own time commitments and the desire to keep helping the car owner. This is something that the sue happy also don't see.

Changing the engine harness is an hour / hour and a half job. All the wires lay where they are supposed to be making the job easier. The throttle body connector might be difficult to get to from what I recall.

I sure hope I'll be driving at 94. That's incredible.

If the 140 is anything like the E420, the upper engine wiring harness is easy to change. It should take about an hour with a screw driver and an Allen wrench. The hardest part is getting the cantilever plug through the hole in firewall. If I recall, it was easier for me to run all the connectors through the hole from the back.

Are you getting any codes on any modules?

No idea if this helps but I had a strange issue with my 95 E300D, its high speed fans would switch on when started from cold and then go out with an intermittent on off dance at approximately 60C on the gauge. Every single time.

It also caused the A/C to cut out.

The high speed fans are controlled by the push button unit - there was a compound problem of wrong wires in the connector pins and one slightly corroded wire in the main grounding point in the cabin.

For your car I have a thought, when the MAF is disconnected the fans kick on high speed in other similar vintage MB and the car is stuck in limp mode. Your fan going high speed may not be a cause but a "result"

Interesting...

I did disconnect the MAF which resulted in a drastic change of throttle response so, I assumed it was functioning. Didn't note if the fans went to high speed or not.
I put together a LED code reader for another W140 years ago. Maybe I can use it and see what I find. Although, If it has shorted wires I'm wondering if it will lead me down the correct path.

Touching anything on this thing worries me after reading about the wiring harness issues.

Yep, she's 94 yrs young and was a psychology professor back in the day, still very sharp. I stopped by one day and found her out washing her MB... barefoot.

Forgot to add that the SRS light comes on during this. Or, at least I witnessed it on once during the limp mode. Not sure if that is related.

It sounds like you have more than one thing going on there. I would pull and record all codes. Then clear them and see which codes come back after driving it.

This may be a shot in the dark, but the coolant temperature sensor on top of the intake manifold (pic) is connected to the engine wire harness. If that connector is shorted, or it shorts when the fans go to high, maybe it's possible that you've found your problem.

If you remove that plug connected to that blue coolant temp sensor with the engine running, your fans should kick into high. I would check the wiring insulation around this sensor. It's not a permanent fix, but if that is causing a short to the rest of the harness it may point you in the right direction.

Often times, a bad engine wiring harness will show up at this sensor first.